1. Field of the Invention
The present invention concerns magnetic recording, and particularly concerns novel and improved magnetic recording members carrying layers of maghemite (gamma Fe.sub.2 O.sub.3) iron oxide crystals, the characteristics of which are adapted in particular to the realization of high-performance magnetic records such as tapes, discs, drums, and striped motion picture film. Additionally, the invention also relates to a novel process for producing such maghemite iron oxide crystals, for producing goethite crystals as precursors thereof, and to novel maghemite crystals as articles of manufacture.
2. The Prior Art
The utilization of magnetic recording in various technical fields is increasing, and efforts are being made to extend more and more the range of its possibilities. It is particularly desired, in the field of sound recording as well as in the field of image recording, to have available magnetic layers that are capable of storing signals of shorter and shorter wavelengths, without sacrificing the quality of reproduction of the signals.
It is known that the end capabilities of magnetic webs and surfaces, such as tapes, discs, drums, and striped motion picture films, for the recording and reproduction of sounds, images or other signals are closely connected with the properties of the magnetizable iron oxides employed.
The most frequently used magnetizable oxide is acicular gamma ferric oxide (maghemite Fe.sub.2 O.sub.3) particles of a length below one micron, obtained from hydrated ferric oxide crystals (goethite FeO.OH). The goethite, after dehydration to alpha Fe.sub.2 O.sub.3 (hematite), and then reduction to Fe.sub.3 O.sub.4 (magnetite), is mildly oxidized to gamma Fe.sub.2 O.sub.3 (maghemite) having a cubic lattice, with the final maghemite particles retaining the dimensions and general appearance of the precursor goethite particles.
Electron micrographs of prior art gamma Fe.sub.2 O.sub.3 particles show that they have on the average a length-to-width ratio, called their acicularity ratio or acicularity, between 5 and 10 to 1. The shape of these particles is often poorly defined, which gives them a rough aspect. Certain shapes suggest the existence of twin crystals. The particle size distribution is heterogeneous, and one notes in particular the presence of fragments of poorly defined shape.
In some samples, a substantial amount of non-ferromagnetic alpha Fe.sub.2 O.sub.3, can be detected along with the ferromagnetic gamma Fe.sub.2 O.sub.3, when examining X-ray diagrams. The presence of alpha Fe.sub.2 O.sub.3 can be explained by local increases in temperature (or hot spots within the mass of magnetite particles) during the oxidation of Fe.sub.3 O.sub.4 into gamma Fe.sub.2 O.sub.3, which reaction is strongly exothermic. Iron oxide such as just described, having a low acicularity ratio and comprising heterogenous particles, will not produce magnetic tapes displaying optimum properties.
Moreover, for certain applications such as studio recordings and cartridges designed for the reproduction of music, magnetic tapes using iron oxides of the prior art do not exhibit as low a level of zero signal or white noise as desired. By white noise is meant the noise which exists in the absence of modulation, but subsequent to high-frequency erasure and recording bias. The level of white noise is reduced when one reduces the size of iron oxide crystals, but this improvement has been generally obtained in the past at the expense of other electroacoustical properties, particularly at low frequencies, and one is forced to compromise.
A procedure for producing iron oxide crystals of improved characteristics is described in copending U.S. application Ser. No. 36,385 filed May 11, 1970, now abandoned, by Bernard-Jean Pingaud, one of us. That application describes dispersing, in the absence of an oxidizing agent, a solution of ferrous salt in an alkaline solution in an amount exceeding the stoichiometric quantity necessary to convert all the iron salt to iron hydroxide, in such a way that the final concentration of alpha FeO.OH is below 15 g/l and that the final concentratin of dissolved alkaline hydroxide is below 60 g/l of solution. Absence of an oxidizing agent is assured by maintaining an atmosphere of inert gas above the reactants. After oxidizing to geothite by blowing air therethrough for 24 hours or more, the dispersion is brought to boiling and boiled for at least 6 hours so as to complete crystallization of the goethite.
When the ferrous hydroxide is precipitated in alkaline medium with only ordinary stirring, a localized precipitation of Fe (OH).sub.2 having an acid character is created adjacent the area where the iron salt enters the alkaline solution, ie., where there is a localized excess of iron sulfate, which permits the forming of goethite only when one oxidizes the intermediate products very slowly. It is one of the reasons why, in the previous Pingaud process, the oxidizing of Fe(OH).sub.2 in alkaline medium must be spread over at least 24 hours so as not to cause the formation of undesirable compounds, such as magnetite or lepidocrocite.